Field of the Invention
The invention relates to a method and a system for determining the topography for reaction signals of an eye, partial surfaces of the retina being cyclically stimulated and, in the process, the total reaction of the eye in the steady state being measured.
The determined topography for the reaction signals of an eye shows the objective sensitivity distribution of the retina and therefore provides information on the visual faculty. Eye diseases, for example glaucoma, can be detected early and assessed with the aid of such an examination. Above all, partial defects of the retina can be established with the aid of such examinations.
There are a multiplicity of examination methods that operate on a subjective basis, that is to say in which the patient examined evaluates the measurement by his statement. In all these methods, the patient is integrated into the measurement by virtue of the fact that he makes the statement as to whether and how he perceives a specific stimulus.
The electroretinogram (ERG) has become established as an objective measuring method; in this case, the reaction signal tapped from the eye with an electrode is displayed and evaluated with its time profile. Single light flashes or cyclic light/dark sequences (flicker ERG) are used for the stimulation. In this case, the mean value is determined for the total retinal area. In the recording of evoked potentials, electrodes are fitted at a specific point on the head, and the measured signal corresponds to the reaction that occurs at the measuring position via nerve cords.
Details on this have been published by J. Jörg and H. Hielscher in the book entitled “Evozierte Potentiale in Klinik und Praxis” [“Evoked Potentials in Hospitals and General Practice”]/Springer Verlag.
In order to determine the topography of the retinal sensitivity, it is fundamentally possible to stimulate partial surfaces of the retina by individual light stimuli assigned to the respective partial surface, and to measure the reaction. Since it is necessary to form mean values over a multiplicity of measurements in order to reduce the noise, this method results in an unacceptably long measurement time.
U.S. Pat. No. 5,382,987 proposes the coupling of an ophthalmoscopic system based on a 3-path Maxwell viewing system for optical examination of the retina of the eye to a perimetric system for determining the visual field, it being possible to measure the spectral sensitivity of a selected portion of the retina with the aid of the electroretinogram for which a stimulus pattern is transferred onto the retina. This method can be used to examine the sensitivity of a single portion, and to examine the overall image of the retina. An unacceptably long examination time would also result here for examining all the portions of the retina by measuring them.
The use of a laser projector provided with an optical modulator for generating a brightness pattern on the retina for measuring the pattern electroretinogram (PERG) was specified by Daniel R. Peters and John Tabora in U.S. Pat. No. 5,233,373. However, it is also possible thereby to examine only ever a selected surface of the retina, and the examination of a plurality of surfaces can be performed only sequentially.
An improved method and the associated system for determining the functional distribution of the reaction over the surface of the retina to stimuli has been specified by R. Richardson in European Patent 0 375 737. Here, the overall reaction of the retina to stimuli in the visual field is recorded, the stimuli being formed by series of patterns whose intensity varies in the horizontal direction and in the vertical direction. As an example, a sinusoidal or cosinusoidal distribution of the intensity is used, and the sensitivity distribution over the surface of the retina can be calculated by inverse transformation from the measured aggregate signals. It is disadvantageous in this method that measuring errors in the determination of the individual coefficients cannot be detected, but that individual measuring errors affect the calculation of the overall distribution function. A further disadvantage in this method is that the resolution must be adapted to the highest density of the sensitivity distribution, although this is present only in a narrowly limited region.
U.S. Pat. No. 5,539,482 specifies a method in which the stimulus pattern comprises a plurality of quadrilaterals of size increasing outward from the center and whose brightness profile is controlled with different frequencies in the range from 10 Hz to 45 Hz. In the example illustrated, 9 quadrilaterals of simultaneously modulated brightness are used, and the evaluation of the measured signal is performed with the aid of the Fourier transformation. It is true that it is possible in this case to detect the influences of the lower Ganglien cell layers by measuring the Nyquist frequency, but determining the Nyquist frequency requires carrying out a plurality of measurements with a different distribution of the modulation frequencies, and the measuring time must be selected to be so long for each frequency distribution that it is possible to determine unambiguous results for the individual frequencies. It is specified that an extension up to 32 “zones” is possible. Consequently, even in the case of this extension the method still has a very low resolution with regard to the partial surfaces that can be examined.
Another method was specified by E. E. Sutter and D. Tran in the journal Vision Research (Great Britain) Vol. 32, No. 3, pages 433 to 446, 1992, with the aid of which relatively good results were achieved. To determine the topography of the ERG components, a digital method is applied in which use is made as stimuli of hexagons whose temporal brightness profiles are controlled by m-sequences. Use is made in this case of 241 hexagons whose size increases outward from the center. Consequently, account is taken of the unequal density of the distribution function. The overall reaction signal of the eye is measured, and the signal profile is calculated for the relevant hexagon by calculating the cross-correlation function with the respective m-sequence. By weighting the signal profile with the mean value for the corresponding region of partial surfaces, interference is reduced in the determination of the amplitude, included in the signal profile, of the useful signal. To perform the measurement, use is made of m-sequences with a length of 65535 steps, which are always offset from one another by 256 steps. Given a display refresh frequency of 67 Hz, the result is an overall measuring period of approximately 16 minutes, the measurement having been subdivided into 32 time segments each being 30 seconds, plus a time for the overlap. A first disadvantage is that the method can provide exact results only to the extent that the reaction function of the retina also reacts linearly to the stimuli. However, this circumstance obtains only partially in the case of an image refresh rate of 67 Hz, and then only, again, given a short after-glow period of the display screen. A further disadvantage of this method consists in that the signal profile must be monitored subjectively, and the associated time segment must be repeated in the event of detectable interference, for example owing to blinking or given contact problems of the electrode. However, the most important disadvantage is that there is no possible way of assessing the intermediate results of the individual time segments, and that it is only at the end of the measurement, that is to say after the measured values over all time segments are present, that it is possible to determine a result and evaluate this result, and so the total measurement must be repeated when interference is not detected.
In U.S. Pat. No. 4,846,567, E. E. Sutter has already specified a basic principle of the method in which a display with a square array of elements that can be activated is used as stimuli, the temporal brightness profiles of the elements being controlled by m-sequences. The calculation of the individual reaction signals is performed with the aid of the cross-correlation function. Here, too, m-sequences of length 216−1=65535 are applied, being offset in each case by 256 steps with reference to one another. It is likewise proposed to subdivide the overall measurement into time segments of approximately 20 to 40 seconds.
A development of this method is specified in German Patent 196 49 858, in which short and corrected m-sequences are used for controlling the light/dark sequences. Here, the individual intermediate results can be evaluated, and it is possible both to achieve a higher degree of effectiveness and an enhanced level of reliability in the evaluation of the results.
The determination of the topography of the retina of the eye from the reaction signals that result in the case of cyclic stimulation in the steady state is possible both with the method specified by E. E. Sutter and with the method specified in German Patent 196 49 858, but has the disadvantage that a poorer signal-to-noise ratio results for the same time segments, since in the case of the use of m-sequences only every second step of the measuring cycle generates a useful signal, but noise is produced in each step of the m-sequence and, in addition, it is first necessary to produce the steady state in each step of the m-sequence before an evaluation can be performed. Thus, it is therefore possible to evaluate only a portion of the reaction signals of a step, and the result in this portion is formed by two measures of noise and one measure of useful signal.